1. Field of the Invention
This invention relates generally to amplifier circuits, and more particularly to an amplifier circuit having offset voltage correction.
2. Prior Art
The offset voltage of an amplifier is generally considered as noise and may be generated as the result of a number of factors such as, for example, mismatch of the active elements or of the load impedances of such an amplifier. The offset voltage of a particular amplifier can be defined as that voltage which is required at the inputs of an equivalent ideal amplifier to produce an output equal to the output of that particular amplifier when its inputs are connected together. That is, if the inputs of a differential amplifier, for example, are connected together, any generated output which is different from zero is equal to the offset voltage of that differential amplifier multiplied by its gain. Accordingly, it can be appreciated that the offset voltage of an amplifier will cause an error in its output signal.
One technique which has been employed in the past for correcting the offset voltage of a differential amplifier involves the use of a circuit for sensing and storing that offset voltage during a first time period and supplying it to the amplifier input in such a manner that it is in opposition to the offset voltage during a second time period when the amplifier is connected to the signal source. Generally, the implementation of this technique involves the disconnection of the inputs of the differential amplifier from the signal source and the subsequent connection of those inputs to one another. The product of the offset voltage and the gain of the amplifier will, therefore, appear at the output of the differential amplifier. By connecting the output of the differential amplifier through an inverter to a capacitor, for example, a voltage which is proportional, but of opposite polarity, to the offset voltage will be stored thereon. With the capacitor connected to the noninverting input of the differential amplifier a unity gain feedback is provided such that the stored voltage will be equal to the offset voltage. When the signal source is subsequently connected to the inverting input of the differential amplifier and the inverter disconnected from the storage capacitor, the offset voltage of the amplifier will be cancelled during signal amplification.
However, the connection of such a capacitor or any other storage element to the noninverting input of the differential amplifier precludes the use of that input for any other purpose. Accordingly, a differential amplifier employing the above described technique for offset correction thereof may not be employed in a number of well known circuit configurations, such as in a buffer amplifier configuration.
The above described offset correction technique requires the periodic disconnection of the signal source from the input of the differential amplifier and the periodic connection of the storage capacitor to its inverted output, such that the stored offset voltage is periodically replenished. Such disconnection of the signal source and replenishment of the storage capacitor causes a disruption in the output signal of the amplifier. That is, the output signal will be chopped whenever the offset voltage stored on the capacitor is being replenished. Such circuits, because of their chopped outputs, are called chopper stabilized amplifiers. Because of the discontinuities in the output signal, a filter must be employed at the output of the amplifier. However, the use of such a filter degrades the output signal and contributes to the cost of the amplifier circuit.
Furthermore, the above described offset correction circuit requires an inverter amplifier in a feedback loop which must have a relatively high frequency response. An inverter having a frequency response which will be sufficient to maintain stable operating conditions of the system will be at least as complex as the amplifier which is being offset corrected and would, therefore, be at least as expensive as such an amplifier. Accordingly, it can be appreciated that such offset correction increases the cost of the amplifier system considerably and degrades its output signal.
It can be appreciated, therefore, that a need exists for an amplifier circuit having offset correction in which both the inverting and the noninverting inputs thereof are available for connection of a signal source thereto. A need also exists for such an offset corrected amplifier which does not require an inverter, particularly one having a relatively high frequency response. Furthermore, such an offset corrected amplifier is needed which will produce an output signal which is not chopped during the offset correction mode thereof.